1. Field of the Invention
The present invention relates generally to the separation of liquids from solid particles contained in a multi-phase reactor effluent. More specifically, the present invention relates to a system and method for separating liquid from catalyst particles used to catalyze reactions. Still more specifically, the present invention relates to a reliable and efficient means to separate liquid from solid particles having magnetic properties.
2. Description of the Related Art
Separation of liquid from solid catalyst material used in multi-phase reactors is of paramount importance to many processes and has been intensely studied. For example, the catalyst-liquid separation is one of the most critical steps in the application of slurry type reactors for Fischer-Tropsch (hereinafter FT) processes. Only if this separation is effective is the application of such reactors successful. Separation techniques typically include settling and filtration. Additionally, combinations thereof have been proposed. Magnetic separation as a stand alone process has been proposed.
Cross-flow filtration is a commonly used method. Mild cross flow filtration has been proposed, and this method claims the cake of catalyst particles formed on the surface of filter media acts as the primary barrier to prevent solids from passing through the filter media thus contaminating the liquid. For example, U.S. Pat. No. 6,929,754 discloses a solid/liquid separation system and method for removing wax products from a slurry used in a Fischer-Tropsch reactor. The preferred embodiments of U.S. Pat. No. 6,929,754 are characterized by a solid/liquid separation system that removes liquid products from a slurry by drawing the fluid across a filter medium composed of a filter cake disposed on a substrate. In the preferred embodiments, the filter cake is desirable and performs the majority of the filtration.
The primary disadvantage of filtration methods is that the filter media is prone to clogging, or plugging by small particles resulting from physical and chemical attrition of the catalyst during use. Filtration media are designed for a certain micrometer rating, say 20 micrometers, so that any particles larger than 20 micrometers will be retained on the surface of the media. Particles smaller than 20 micrometers will travel through the media and may exit or get stuck within the pores of the filter medium due to agglomeration, shape, and other factors. Although a backwash method may be used to unplug the medium, with time on stream, backwash may become less effective and eventually the filter elements must be removed from the system and replaced. Fischer-Tropsch catalysts, typically iron-based or cobalt-based, are prone to attrition. Typical fresh catalyst particles are in the range of from 20 micrometers to 100 micrometers. Attrition leads to the formation of particles less than 20 micrometers in size, with some particles in the sub-micron size range. These smaller particles may either clog or plug the filter media, or change the cake composition in such a way that the filter media becomes impermeable and compacted. Compact cakes cause the need for higher pressure drop across the media to get the same volume of liquid across the filter. This leads to a vicious cycle of higher pressure drop leading to an even more compacted cake and/or media plugging which will render the system ineffective.
Settling is another method proposed to separate solid material from liquids in FT processes and other multi-phase reactor systems. Typical settlers are of two types: vertical settlers and inclined settlers (also known as lamellar settlers). U.S. Pat. No. 6,833,078 discloses a solid/liquid separation system and methods for separating liquid products from catalyst fines from a slurry used in a Fischer-Tropsch reactor. A settling system continuously or intermittently removes catalyst fines from the slurry and is coupled with catalyst/liquid separation system that separates liquid products from the slurry.
U.S. Pat. No. 6,068,760 discloses a catalyst/wax separation device for slurry Fischer-Tropsch reactor whereby catalyst particles are separated from the wax in a Fischer-Tropsch reactor by feeding a portion of the reactor slurry to a dynamic settler which does not require any pump. As the slurry flows down a pipe in the center of the settler, the slurry flows into the surrounding annular region at the bottom of the settler. The heavier catalyst particles settle down and are removed as the slurry at the bottom of the settler is recycled back to the reactor. The wax rises up in the annular section and this clarified wax is removed by a wax outlet pipe.
In U.S. Pat. No. 6,730,221, Bohn et al. describe a method whereby catalyst particles are separated from the wax in a slurry reactor by feeding a portion of the slurry to a dynamic settler. Heavier catalyst particles settle and are removed as the slurry at the bottom of the settler is recycled back to the reactor. Clarified wax is removed at the top of the settler. A multi-channel baffle prevents turbulence, improving retention of the desired heavier catalyst particles.
The design of dynamic inclined settlers is such that they allow higher liquid removal rates than similarly sized vertical settlers. In U.S. Pat. No. 7,078,439, Odueyungbo, et al. Jul. 18, 2006 disclose systems and methods for catalyst/hydrocarbon product separation from a FT product slurry. The preferred embodiments in U.S. Pat. No. 7,078,439 are characterized by a separation system that uses a sedimentation chamber, which contains at least one inclined channel that enhances the settling of particles within the slurry. The inclined channel may be provided by a structure selected from the group consisting of tube, pipe, conduit, sheets, trays, walls, plates, and combinations thereof.
In settlers, liquid is typically withdrawn from the top section of the settler. The particle settling and removal rates are dependent on particle settling velocity, which is dependent on particle diameter. The design of settlers is to remove a specified range of particle sizes or larger. Particles in the liquid change size due to attrition over time, as they decrease in size, they leave the settler with the liquid withdrawn, thus contaminating the liquid. This renders the settler which is designed for a particular range of solid particles ineffective. In a FT process, any time catalyst particles leave the reactor, it not only contaminates the liquid product, but decreases the catalyst inventory in the reactor; both may be detrimental for the process economics. Another problem with settlers is that mixing due to convective flow may occur within the settler, lifting particles upward and contaminating the overflow (i.e. the liquid withdrawn from the top section of the vessel).
Reduction of solid catalyst particle size with time in a multiphase reactor or slurry bubble column reactor (due to physical and/or chemical attrition) causes a settler with a certain particle size removal to become ineffective. At near constant operating conditions (e.g. pressure, temperature, liquid composition, etc.), a settler can be designed to remove a certain amount of liquids allowing for the solids to settle and follow the slurry path (underflow of the settler) to get a liquid as the overflow of the settler almost free of solid particles. This design works provided the minimum size of the particles for which the settler was designed remains constant. If the minimum size starts to shift to smaller particles, complete separation of solid particles will not occur and some particles will leave with the liquid in the overflow of the settler.
Magnetic separation has been proposed as a stand alone system to separate solids and liquids in FT reactor systems. This system consists of passing the slurry containing liquids and solids to be separated through a vessel with magnetized walls. The solids with magnetic properties will accumulate on or near the walls or along magnetic fields created inside the settler vessel, fall vertically to the bottom of the vessel, and continue to travel in the direction of the slurry stream. Thus the solids can be separated from the liquids which can be withdrawn from the top of the vessel. This technique has been shown to be effective for the removal of solid particulates on the small micron to sub-micron scale range.
Accordingly, a need exists for an efficient and reliable system and method for separating solid catalyst particles from a slurry. The system and method should desirably continue functioning even when minimum particle size shifts to smaller particle size due to catalyst attrition.